The mite Varroa destructor (“Varroa”) is a honey bee (Apis mellifera) ectoparasite that causes extensive colony losses. Varroa destroys bee colonies for two main reasons: the mites feed on the bee's hemolymph, thereby weakening them; and they transmit pathogens, such as the Israel acute paralysis virus (IAPV) and the deformed wing virus, that can cause crippling bee diseases. The mites are not easy to detect, and they can cause a seemingly healthy and productive honey bee colony to collapse within a few weeks.
Varroa is now considered to be one of the most significant threats to apiculture around the world. For example, in Canada, Varroa was first detected in New Brunswick in 1989. In 2013, all provinces in Canada had the mite, and reported mites that were resistant to the treatments used to combat them. In Israel, Varroa was first detected in 1984. Indeed, since Varroa jumped hosts from the Asian honey bee (Apis cerana) to the European honey bee (Apis mellifera), the mites have spread nearly worldwide. As of 2010, only a few islands or isolated regions, such as Australia, the Southern part of New Zealand, Newfoundland and Madagascar do not have the mite. Furthermore, due to local warm climatic conditions, Varroa reproduction continues year round, making it impossible to grow bees without efficient Varroa control.
Several different synthetic acaricides have been implemented against the Varroa mite. However, over the years, Varroa developed resistance to pyrethroids and recently to Coumaphos (CheckMite+), rendering these acaricides ineffective and thus leaving local apiculture in a very problematic condition.
Currently available chemical control methods have certain disadvantages. For example, amitraz (an acaricide), causes problems of resistant Varroa and honey contamination; fluvalinate (e.g., Apistan® strips), a pyrethroid, causes problems of resistant Varroa; coumaphos (e.g., CheckMite+™ strips, an organophosphate) causes problems of resistant Varroa, and its interaction with fluvalinate causes bee mortality; natural hops extract (e.g., Hopguard®) causes potential Varroa resistance; thyme essential oil also causes potential Varroa resistance; and formic acid and oxalic acid can cause bee mortality and have variable success in control of low infestation levels, but are not effective in severe infestations and are problematic for implementation in areas with hot climate.
Current available physical control methods also have drawbacks. For example, fine sugar powder can be used to abrade Varroa cuticles, however, it has highly variable effectiveness in controlling Varroa, ranging from no control to moderate Varroa reductions. As another example, a frame heater can be used to exploit the differential heat sensitivity of Varroa and brood (Varroa is more susceptible to heat than the bee brood), however, its use is very labor intensive and disruptive to the colony. Drone brood frames can be actively removed and destroyed, particularly because Varroa is more prevalent on drone brood. However, drone brood frame removal and destruction are only effective for minor mite control, are very labor intensive and do not completely remove the mites. As yet another example, bottom board excluders and sticky boards reduce the likelihood that Varroa that have fallen off bees can re-enter the hive. However, bottom board excluders and sticky boards only remove some of the mites.
As discussed above, available disruptive compounds and methods generally do not provide high efficiency protection against Varroa. However, it is believed that chemical cues can play an important role in modulating host-parasite interactions. For example, parasites often eavesdrop on their host's chemical signals, and rely on these signals for host detection and choice. Parasitism of social insects is an especially complex case, as numerous chemical signals, known as semiochemicals, are important for the function of the society, including its protection from inquilines. Although semiochemicals are well-known tools in pest management, in the enclosed and crowded environment of the colony, the proximity between the host and parasites presents an obstacle for parasite (e.g., Varroa) control without damaging the host (e.g., Apis mellifera).
Varroa life cycle can be generally divided into two main phases: a phoretic phase, in which the Varroa is parasitizing an adult bee, and a reproductive phase, in which the Varroa is reproducing within a sealed brood cell. Between these phases Varroa move freely on the surface of the comb. The entrance of the fertilized Varroa female into a brood cell is synchronized with the developmental stage of the honey bee larvae, and occurs just before the cell is capped. It is believed that semiochemicals play a major role in the host-finding and preference of Varroa. For example, in laboratory bioassays, Varroa has been shown to discriminate between bees from different task groups, and to prefer a nurse over a forager. The host preference is apparently based on compounds with low volatility, such as cuticular hydrocarbons, and on volatile compounds emitted by the honey bees and their environment (such as larval food and brood pheromone). Despite much progress in the identification of host olfactory cues guiding Varroa, neither effective attractants nor repellents have been found so far. In view of limited success in exploiting hive semiochemicals in Varroa control, the use of synthetic disruptive compounds can be another approach for confronting the mite.
Furthermore, the olfactory organ of the Varroa is located on the distal part of its forelegs, analogous to the sensory pit (Haller's organ) found in ticks. Although chemosensory sensilla in a mite's sensory pit appear similar to those described in insects, not much is known about the mechanism behind odorant detection in mites in general and Varroa in particular. Only a few attempts of electrophysiological recordings from the Varroa foreleg have been mentioned in the literature.
Accordingly, there is a need for compounds that disrupt the Varroa-honey bee interaction by targeting Varroa's olfactory system, and for methods of evaluating Varroa's sensitivity to the compounds. Such compounds should confuse Varroa while minimally disrupting honey bee communication in the colony. The present disclosure seeks to fulfill these needs and provides further related advantages.